Conventionally, in a sterilization process for food in inert gas-filled packages, food which has been cooked up to a certain stage is put into a bag (“retort pouch”) formed of highly gas-impermeable laminated sheet material; the bag is hermetically sealed as by heat sealing after replacing the air in the bag with an inert gas; the bag thus sealed is loaded into a sterilization tank; and a hot water or heated steam is fed into the sterilization tank to sterilize the food in the bag.
To enhance the sterilization effect as well as to maintain the taste, shape and quality of the food, it is desirable that the sterilization be performed at a higher temperature and for a short period of time. According to a certain heat sterilization standard, the temperature of 120° C. is generally maintained for four minutes at the heart of the food (F value-4).
In a conventional sterilization tank, the food in inert gas-filled packages is immersed in hot water for a certain period of time, or otherwise, the food is subjected to sprays of hot water or heated steam for a certain period of time in a certain pressurized condition. However, as shown in FIG. 9, these methods cannot raise the core temperature of the food in inert gas-filled packages readily to a predetermined sterilization temperature. Also, temperature control tends to be inaccurate. Accordingly, it is necessary to increase the heating time equivalently if it is desired to enhance the sterilization effect. As a matter of fact, the sterilization is performed in the neighborhood of 120° C. for about 60 minutes.
This can provide for adequate sterilization of a major portion of bacteria which cause food poisoning; however, some bacteria can survive the treatment. Accordingly, in order to enhance the sterilization effect 3-percent hydrogen peroxide sorbic acid or the like are used additionally, or packages are sealed under a vacuum or an inert gas atmosphere.
During the sterilization process, the food is kept under a heated condition for an extended period of time so that the food sometimes acquires the smell characteristic of the retort pouch with its organoleptic properties deteriorating. Some foods, which need to be shipped in a semi-cooked condition, tend to be cooked over beyond that condition. These provide limitations on quality control of foods in inert gas-filled packages.
The present invention overcomes the above-stated problems of the prior art by providing an inert gas-filled cooking process which can sterilize foods in inert gas-filled packages rapidly and effectively without deteriorating food quality by raising the temperature of the hot water to be applied onto the food packages in the sterilization tank.